The present invention relates generally to control circuits and more particularly to a variable speed control circuit for an encoderless, brushless, direct current motor. Many conventional, brushless, dirrect current ("d.c.") motors have a rotor formed of one or more permanent magnets and a stator employing two or more phase windings. Typically, the stator circumferentially encloses the rotor in spaced relationship. The currents in the stator phases are sequentially controlled in relation to the rotor position. The magnetic flux therefrom interacts with the flux of the permanent magnet rotor, resulting in a force exerted upon the rotor to cause rotation.
Also well known is a brushless d.c. motor of this general type in which there are three phase windings that are "Y" connected. These windings are coupled to the power source through a bridge arrangement of power transistors.
Numerous methods are known in the art for sensing the location and rotational speed of a rotor. Such methods typically involve additional components, such as optical or Hall effect sensors, which generally reside within the motor housing. Such arrangements detect a passing marker or shutter or magnetic field on the rotor.
Such sensors are expensive to manufacture and precisely mount within the motor. Moreover, additional leads must typically be wired between the sensor and the control circuit. The sensor is located within the interior of the motor, and the control circuit, which controls the activation of the windings, is remotely located.
Some sensors and control circuits require only one additional lead between the control circuit and the common connection of the motor windings. Nonetheless, even the addition of one wire adds significantly to the time and expense required to manufacture a motor.
Other techniques for sensing the position or speed of the rotor, or armature, include the use of a shaft rotation tachometer or other speed sensor. Such sensors are frequently not effective for all armature positions or under varying load conditions or when the motor is starting.
Other sensors used with brushless direct current motors fail to allow the speed of the motor to be regulated over widely varying temperature conditions, or do not allow the speed of the motor to be varied over a wide range. Still others are slow to adjust the power applied to the motor in the case of changing load, temperature, or input signals. Finally, other schemes do not provide for changing the voltages applied to the windings (commutation) at optimum times for all load, speed, and temperature conditions or for reverse movement of the motor armature.